Whole-rock geochemistry of the Hili Manu peridotite,East Timor: implications for the origin of Timor ophiolites ∗

The Hili Manu peridotite occupies a key position at the outer limit of continental crust on the north coast of East Timor. Most models for the tectonic evolution of the Outer Banda Arc interpret peridotite bodies on Timor, such as Hili Manu, as fragments of young oceanic lithosphere from the Banda Arc (upper plate). However, recent workers have used major-element geochemistry to argue that the peridotite bodies on Timor were derived from the Australian subcontinental lithosphere. Our major, trace and isotopic geochemical study of the Hili Manu peridotite body supports a supra-subduction origin from either a forearc or backarc position for the Hili Manu peridotite. In particular, the wide range in Nd and Sr isotopic compositions, overlapping that of arc volcanics from the Sunda – Banda Island arc, and highly fractionated Nb/Ta values indicate a supra-subduction setting. As there is no evidence for subduction beneath the rifted Australian continental margin, it is unlikely that the Hili Manu peridotite is Australian subcontinental lithosphere. This result, along with the clear supra-subduction setting of the Ocuzzi peridotite and associated volcanics in West Timor, gives support to the interpretation that the Miocene collision between the Banda Arc and the Australian continental margin has produced widespread ‘Cordilleran’-style ophiolites on Timor.     

Thrust tectonics in Timor

Recently it has been argued that the structure of the island of Timor can be interpreted without invoking the concept of major overthrust‐faulting. Using evidence from the Maubisse area of eastern Timor, Grady (1975) has suggested that the relationship between contiguous rock units in that area may be interpreted either as an unconformity or as steeply dipping fault‐planes. In the present account interpretations of the structure of Timor are reviewed and the concept of overthrusting is reconsidered. It is concluded that the structure may only be interpreted in terms of a series of overlapping thrust slices resting on folded sediments of the Australian continental shelf. The lowest thrust sheet, the Kolbano thrust sheet is composed of internally deformed deep‐water calcilutites. It is followed to the north by the Lolotoi thrust sheet, made up of a complex group of crystalline rocks varying from granulite to slate, together with unmetamorphosed ophiolites, clastic sediments, and massive Miocene limestones. Overlying this group to the north is the Maubisse‐Aileu thrust sheet composed of Permian crinoidal limestones and volcanics in the south, passing northwards into shales and sandstones. Within this unit there is also a marked increase in deformation and metamorphism from south to north. Slates in the south pass into mica schists, psammites, marbles, and hornblende schists of the amphibolite facies on the north coast of eastern Timor near Manatutu. A further thrust‐slice composed of ophiolites rests on this thrust unit on the north coast of western Timor between Wini and Atapupu.

The composition, structural state, and metamorphic grade of the rocks composing each of these thrust slices is described. The detailed relationships of the thrust units, including those of the Maubisse area, in the neighbourhood of the thrust planes is reconsidered. The case for the concept of major overthrusting is restated, both from regional considerations and from new evidence obtained during recent field mapping. This interpretation is placed in the context of a collision between the Australian continental margin and a detached portion of the Asiatic continental margin during the Cainozoic Era.     

THICKNESS OF THE CRUST AND ITS RELATIONSHIP TO RELIEF AND GRAVITY ANOMALIES

The effect of different crustal thickness on a regional gravity field may be differentiated, as a first approximation, into-three layers: 1) sedimentary, 2) granitic, and 3) basaltic. The study of complex “wave pictures” obtained in deep seismic sounding has lead to differentiation of the crust as continental, oceanic, and transitional, with a general relationship existing between the surface tectonics of the crust and its deeper structures. The crust is thickest in the mountain regions (40 km-80 km) as against an average for the platforms of about 25 km-35 km. It appears that there are two particularly conspicuous gravity and seismic discontinuities in the crust; one between the sedimentary mantle and the so-called crystalline layer and the other between the latter and the M surface. Tentative estimations of crustal thickness are as follows: the Russian Platform and the north of the western Siberian Platform; 30 km-34 km; the Black Sea about 24 km; the entire south, southeast and east of the U. S. S. R. are marked by greater depth with the Pamirs having a thickness of over 70 km; in the Caucasus the M surface lies below 45 km; in the Northern Kazakhstan the crust is 34 km-36 km thick; in the Altay thickness of around 50 km are indicated; in the Eurasian continent, Tibet has the thickest crust, the gravity minimum indicating about 85 km; in the Verkoyansk region the M surface is over 43 km. Large areas of the Arctic Ocean is occupied by the shelf with a thickness similar to that in the north of the country. This suggests that a considerable stretch of the ocean adjacent to the northern shores of the U. S. S. R. has a continental type. The crust thins rapidly to the north to about 10 km. Along the Pacific coast the M surface is about 33 km, the shelf zone is rather narrow including the Sea of Okhotsk. Toward the ocean and the Kuriles the crust thins rapidly to 10 km. -- C. E. Sears.     

Deformation and metamorphism of the Aileu Formation,north coast,East Timor and its tectonic significance

The large block of metamorphic rocks along the north coast of East Timor is of special interest as it occurs at the boundary between continental and oceanic crust in an island arc-continent collision zone. A detailed study of the structure and metamorphic history of 400 km² of this formation showed it has a complex history of penetrative deformation but the structure is coherent.Pelites, psammites and limestones interlayered with dolerites and amphibolites have been metamorphosed in a medium pressure environment. They now form a metamorphic province zoned from low greenschist facies in the southwest to upper amphibolite facies in the east. The earliest recognised deformation phase predated the metamorphism and produced a widespread layer—parallel schistosity but no recognisable folds. The second deformation phase post-dated the metamorphic maximum and micropetrological evidence indicates a gradual cooling during this event. This deformation produced tight folds with an axial plane schistosity and transposed the earlier structures. The progressively weaker third and fourth phases developed crenulation cleavages and related folds, under greenschist facies conditions. Open, fifth phase, macroscopic folds were probably synchronous with strike slip faulting parallel to the north coast. Later dip slip faulting juxtaposed the Aileu Formation with Permian and Mesozoic sediments of very low metamorphic grade.Reconnaissance K/Ar radiometric dating using hornblende and biotite showed the prograde metamorphic maximum occurred before 11 Ma ago and implies that the second, and strongest, deformation phase occurred in the late Miocene. This young age establishes the relationship of the deformation events to the collision between Australia and the Inner Banda Arc.The proposed models for the structure of Timor must be modified to fit the deformation history of the Aileu Formation. If Timor is essentially autochthonous, the Aileu Formation was probably deposited in a Palaeozoic graben and the metamorphic maximum may have occurred in the Jurassic. The overthrusting models must be modified in the light of the close correlation in time between penetrative deformation and emplacement of the proposed thrust sheets. The analogy proposed between Timor and ‘normal’ convergent margins is not supported but it may be possible to draw analogies with the Molucca Sea.     

An interpretation of some magnetic data off the west coast of Ireland

A total field magnetic anomaly contour map of the continental margin west of Ireland is described by reference to geological structure previously established by seismic reflection profiling. The western Irish Mainland Shelf, Porcupine Seabight Trough and Porcupine Ridge were previously defined as major north-south elements in the structure of the margin. Each is readily distinguished on the magnetic anomaly contour map. To the north these features terminate against, or merge with, the east-west Slyne Ridge. A pronounced change in the magnetic anomaly pattern at about 53°N suggests that the southern limit of this Ridge coincides with a major geological discontinuity extending from near the coast of Ireland to the continental slope, some 320 km to the west. The magnetic anomaly contour map gives general support to the idea that the Slyne Ridge and Porcupine Ridge represent submerged blocks of marginal continental crust, and that the Porcupine Seabight Trough is founded upon crust significantly different in character.     

Gravity study of the Djibouti area

In March and April 1972, 380 gravity stations were established in the T.F.A.I. The data were reduced to Bouguer anomalies of 5 mGal isolines. Qualitative and quantitative interpretation based on the gravity map, on seismic data and on the magnetics and physiography of the area revealed that: The crust of the Gulf of Tadjura and the central part of the T.F.A.I. is strongly oceanized and is the direct continuation of the Sheba Ridge.To the north of the Gulf, at the Dankali Mountains, the crust increases in thickness and most probably contains sialic fragments, indicating the continuation of the Danakil Chains in the T.F.A.I.To the south, the structure is bordered by a continental block, the Aysha Horst. The oceanization is concentrated only in the area of deep injections, marked by gravity maxima, whereas the rest of the area is to be described as sub-continental. The crust is attenuated from south (Aysha Horst) to north (Red Sea), from 30 to 15 km in thickness.The pattern of the gravity anomalies shows clearly liniations only along the coasts of the Gulf of Tadjura and the Straits of Bab el Mandeb. Inland the field breaks up into relative minima and maxima, indicating the fragmentation of the crust and the ‘triple junction’ nature of the area.The tectonic process is that of extension, with normal faults having maximum displacements at the northern border of the Gulf of Tadjura.The uppermost mantle has low velocity and density values due to thermal processes in the expanding zone. The state of the upper-mantle material must be that of partial melting due to high temperatures of the order of 800° to 1,000° C at about 15 km depth.     

The Levantine Basin—crustal structure and origin

The origin of the Levantine Basin in the Southeastern Mediterranean Sea is related to the opening of the Neo-Tethys. The nature of its crust has been debated for decades. Therefore, we conducted a geophysical experiment in the Levantine Basin. We recorded two refraction seismic lines with 19 and 20 ocean bottom hydrophones, respectively, and developed velocity models. Additional seismic reflection data yield structural information about the upper layers in the first few kilometers. The crystalline basement in the Levantine Basin consists of two layers with a P-wave velocity of 6.0–6.4 km/s in the upper and 6.5–6.9 km/s in the lower crust. Towards the center of the basin, the Moho depth decreases from 27 to 22 km. Local variations of the velocity gradient can be attributed to previously postulated shear zones like the Pelusium Line, the Damietta–Latakia Line and the Baltim–Hecateus Line. Both layers of the crystalline crust are continuous and no indication for a transition from continental to oceanic crust is observed. These results are confirmed by gravity data. Comparison with other seismic refraction studies in prolongation of our profiles under Israel and Jordan and in the Mediterranean Sea near Greece and Sardinia reveal similarities between the crust in the Levantine Basin and thinned continental crust, which is found in that region. The presence of thinned continental crust under the Levantine Basin is therefore suggested. A β-factor of 2.3–3 is estimated. Based on these findings, we conclude that sea-floor spreading in the Eastern Mediterranean Sea only occurred north of the Eratosthenes Seamount, and the oceanic crust was later subducted at the Cyprus Arc.     

Crustal structure of the Levant Basin, eastern Mediterranean

A seismic refraction/wide-angle reflection experiment was undertaken in the Levant Basin, eastern Mediterranean. Two roughly east–west profiles extend from the continental shelf of Israel toward the Levant Basin. The northern profile crosses the Eratosthenes Seamount and the southern profile crosses several distinct magnetic anomalies. The marine operation used 16 ocean bottom seismometers deployed along the profiles with an air gun array and explosive charges as energy sources. The results of this study strongly suggest the existence of oceanic crust under portions of the Levant Basin and continental crust under the Eratosthenes Seamount. The seismic refraction data also indicate a large sedimentary sequence, 10–14 km thick, in the Levant Basin and below the Levant continental margin. Assuming the crust is of Cretaceous age, this gives a fairly high sedimentation rate. The sequence can be divided into several units. A prominent unit is the 4.2 km/s layer, which is probably composed of the Messinian evaporites. Overlying the evaporitic layer are layers composed of Plio–Pleistocene sediments, whose velocity is 2.0 km/s. The refraction profiles and gravity and magnetic models indicate that a transition from a two layer continental to a single-layer oceanic crust takes place along the Levant margin. The transition in the structure along the southern profile is located beyond the continental margin and it is quite gradual. The northern profile, north of the Carmel structure, presents a different structure. The continental crust is much thinner there and the transition in the crustal structure is more rapid. The crustal thinning begins under western Galilee and terminates at the continental slope. The results of the present study indicate that the Levant Basin is composed of distinct crustal units and that the Levant continental margin is divided into at least two provinces of different crustal structure.     

壳幔物质流变的底辟作用孕育大地震——卫星重力资料的证据

根据2-180阶与2-360阶的数字全球模型恢复的卫星重力场数据,主要源于地下18~36 km深处的密度变化,多数大地震的震源也正是位于该层段。 因此卫星重力场资料对于地震研究有很大的意义。笔者详细分析了中国汶川、墨脱、仲巴、伽师和台湾等地的卫星重力资料(这些地区在近几十年来均发生过6级以上的大地震),惊奇地发现,在这些地区,卫星重力场的样式具有极大的相似性,它们表现在:(1)卫星重力场的正负异常值成对出现;(2)震中通常位于重力场峰值间的陡梯度带;(3)由低阶到高阶的重力场正异常的峰值增大而宽度变窄;(4)正、负峰值间的距离随所处的大地构造背景而变; (5)不同应力场壳幔物质底辟的样式不同,从地质上看,高密度壳幔物质的流变底辟作用,可以合理地解释这种独特的样式变化。文中提出高密度生长指数的概念,用以度量密度由深部到浅部的增加。岩石圈断裂带、深大断裂带及多层次滑脱层是壳幔高密度塑性物质底辟活动的优势通道。壳幔物质的非均质性反映了大陆板块构造动力学过程的复杂性。     

南海边缘海减薄陆壳成因剖析

南海中央海盆南、北两侧陆缘分布着面积较广的减薄陆壳,正确认识海盆减薄陆壳的成因是研究南海构造演化的重要一环.通过分析基于地壳伸展因子公式计算的南海地壳拉张伸展特征和解释中生代以来的陆壳隆升特征等,证实晚中生代以来至渐新世末,该区不仅发生了地壳拉张伸展作用,还发生了较长期的地壳隆升挤压作用,致使酸性侵入岩出露地表,减薄陆壳区的上地壳厚薄分布不均.始新世南海南部发育海陆过渡相和海相沉积、北部仅为陆相沉积,暗示始新世南海古地理格局是南、北陆缘具有不同沉积环境的盆地群,二者之间应该被隆起所隔.这些地质现象说明该区地壳隆升剥蚀与地壳拉张伸展活动时间有较长的重叠.南海中央海盆两侧减薄陆壳的成因不仅仅是地壳拉张伸展所致,而是拉张伸展与隆升剥蚀共同作用的结果,因此可以认为在曾经发生了地壳隆升挤压而遭受长期剥蚀的区域,如果用全地壳伸展因子的公式来估算地壳拉张伸展程度,将得出错误的结论.      

Continent — Island arc collision in the Banda Arc

Acoustical structure of seismic profiles, and morphology of the Timor—Tanimbar—Ceram troughs and adjacent slopes of the outer Banda Arc, show remarkable similarities to equivalent parameters of many arcs subducting oceanic lithosphere and sediments, despite the fact that the outer Banda Arc is underlain by continental crust continuous with that of the colliding Australian craton. Such similarities include diffractions and anticlinal folds at the toe of the inner slope of the Timor—Tanimbar—Ceram troughs, which could be interpreted as thrust slices and thrust folds. Slope basins comprising sediments obviously dammed behind acoustic basement highs are also common on the trough inner slope, with some basins containing strata adjacent to the highs dipping away from the trough. Ridges and basins occur on the trough inner slope oriented parallel to the trough trend, and a slab continuous with down-bowed continental margin can often be detected a considerable distance in from the trough below the inner slope. On face value these observations are compatible with a mechanism of underthrusting by Australian and New Guinea crust with consequent imbrication and accretion of packages of off-scraped sediments. However, they may also be explained as possible outward-directed gravity slides of nappes displaced from uplifted inner portions of the arc, similar to the published structural interpretation of at least the eastern portion of the neighbouring, closely related New Guinea Fold Belt. It is shown that the weight of marine geological and geophysical evidence, including the alignment with the oceanic Indonesian Arc, the gravity anomalies, and the persistence of the various morphological and structural entities around the arc, favours subduction in the Timor—Tanimbar—Ceram troughs rather than massive gravity sliding towards the troughs. By this working model the outer Banda Arc would be the accretionary prism of a subduction zone which was formerly in an ocean-crust setting but since Pliocene has been interacting with continental lithosphere. If its structural evolution is analogous to that of the New Guinea orogenic belt, then the Banda Arc has not yet reached the stage of major, foreland-directed gravity slides. The proposed structural model for the Banda Arc is at variance with some but not all structural interpretations of the island of Timor, which is an emergent portion of the outer arc. Further critical studies are obviously required, both in marine and terrestrial areas, to resolve this impasse.     

Ocean trench blocked and obliterated by Banda forearc collision with Australian proximal continental slope

The bathymetry and abrupt changes in earthquake seismicity around the eastern end of the Java Trench suggest it is now blocked south–east of Sumba by the Australian, Jurassic-rifted, continental margin forming the largely submarine Roti–Savu Ridge. Plate reconstructions have demonstrated that from at least 45 Ma the Java Trench continued far to the east of Sumba. From about 12 Ma the eastern part of the Java Trench (called Banda Trench) continued as the active plate boundary, located between what was to become Timor Island, then part of the Australian proximal continental slope, and the Banda Volcanic Arc. This Banda Trench began to be obliterated by continental margin-arc collision between about 3.5 and 2 Ma.The present position of the defunct Banda Trench can be located by use of plate reconstructions, earthquake seismology, deep reflection seismology, DSDP 262 results and geological mapping as being buried under the para-autochthon below the foothills of southern Timor. Locating the former trench guides the location of the apparently missing large southern part of the Banda forearc that was carried over the Australian continental margin during the final stage of the period of subduction of that continental margin that lasted from about 12 Ma to about 3.5 Ma.Tectonic collision is defined and distinguished from subduction and rollback. Collision in the southern part of the Banda Arc was initiated when the overriding forearc basement of the upper plate reached the proximal part of the Australian continental slope of the lower plate, and subduction stopped. Collision is characterised by fold and thrust deformation associated with the development of structurally high decollements. This collision deformed the basement and cover of the forearc accretionary prism of the upper plate with part of the unsubducted Australian cover rock sequences from the lower plate. Together with parts of the forearc basement they now form the exposed Banda orogen. The conversion of the northern flank of the Timor Trough from being the distal part of the Banda forearc accretionary prism, carried over the Australian continental margin, into a foreland basin was initiated by the cessation of subduction and simultaneous onset of collisional tectonics.This reinterpretation of the locked eastern end of the Java Trench proposes that, from its termination south of Sumba to at least as far east as Timor, and probably far beyond, the Java-Banda Trench and forearc overrode the subducting Australian proximal continental slope, locally to within 60 km of the shelf break. Part of the proximal forearc's accretionary prism together with part of the proximal continental slope cover sequence were detached and thrust northwards over the Java-Banda Trench and forearc by up to 80 km along the southwards dipping Savu Thrust and Wetar Suture. These reinterpretations explain the present absence of any discernible subduction ocean trench in the southern Banda Arc and the narrowness of the forearc, reduced to 30 km at Atauro, north of East Timor.     

大陆块体地球化学边界与成矿——兼论中国东秦岭金属矿集区的构造控制

地球块体不均一性边界存在地球化学急变带 ,控制大型矿床的分布 ;深部构造或隐性的构造面与地壳浅表形成的矿集区有明显的空间和形成机制耦合关系。地球化学急变带在大陆内部往往沿着一些古大陆边缘展布 ,与地壳下部和上地幔的深部构造或扩展到地幔的不连续面具有相联系的空间组合关系 ,反映壳幔相互作用对深部构造效应和大规模成矿热流体的控制。东秦岭金属矿化集中区的大型或超大型矿床沿古大陆边缘产出 ,并受地球化学急变带与地球物理梯度带交叉效应控制 ,揭示了深部构造对大型矿集区的制约。     

Crustal structure of Fiordland, New Zealand

A generalised crustal structure of Fiordland is proposed.Detailed mapping in part of Western Fiordland has led to the recognition of a basement granulite facies lower crustal material, probably Precambrian in age) separated by a regional thrust zone from a cover sequence (amphibolite facies gneisses, of Lower Paleozoic age). With the recognition of the basement—cover relationship and the aid of aeromagnetic anomalies Fiordland has been divided into four, generally north-northeast trending, regions. The Western Fiordland region is composed chiefly of basement rocks. The Central Fiordland and Southwestern Fiordland regions are made up predominantly of amphibolite and greenschist-facies metasediments and gneissic granodiorites of the cover sequence, which in Central Fiordland have a regional dip to the east, off the basement. The Eastern Fiordland region is characterised by a series of basic, intermediate and acid intrusive rocks. The more prominent magnetic anomalies in Eastern Fiordland, Southwestern Fiordland, and a large anomaly off the coast of Western Fiordland, are all considered to be caused by intrusive bodies. The presence of a positive gravity anomaly over Western Fiordland, coupled with a gravity low offshore, is consistent with the lower crust being uplifted and exposed in this area. Continuing shallow and intermediate-depth seismic activity beneath Fiordland, as well as the large size of the gravity anomaly, suggest that tectonic forces are currently acting to maintain Western Fiordland at its unusually high level.Fiordland thus displays a cross-section of continental crust: Precambrian(?) metaigneous granulites in the lower crust; Lower Paleozoic metasedimentary amphibolitefacies gneisses and melted equivalents in the middle crust; Mesozoic intrusives, and overlying Cretaceous and Tertiary sediments in the upper crust.     

The significance of the metamorphic rocks of Timor in the development of the Banda Arc, Eastern Indonesia

The metamorphic rocks of Timor are reinterpreted in the light of reconnaissance mapping of the whole island. All metamorphic rocks that crop out in Timor are allochthonous. Several metamorphic massifs are reported for the first time, the outline of others is revised. On the basis of their grade, three distinct groups can be mapped: lustrous slate, amphibolite-serpentinite, and a granulite-amphibolite-greenschist complex. Each group has distinctive structural relations to other allochthonous elements. The granulite facies meta-anorthosite in Timor must have originated near the boundary between the continental mantle and the crust. These and related high-grade metamorphic rocks may represent slices of an ancient Asian continental basement. These rocks imply that the history of the Mesozoic-Cinozoic fold belt of the Outer Banda Arc extends into the Precambrian Era. The metamorphic rocks of Seram appear to be remarkably similar to those of Timor in grade, distribution and structural relations. The overthrust directions of the metamorphic rocks in Timor is southwards, in Seram it is northwards. As the islands are separated by the 4–5 km deep Banda Sea, these directly opposite thrusts may be explained in terms of the Banda Arc acquiring its sinuosity after the emplacement of the metamorphic rocks.     

Some Palaeozoic‐Mesozoic stratigraphic‐structural relationships in east timor and their significance in the tectonics of timor

An ‘autochthon’ model for the tectonic development of Timor is suggested, based on observations of Palaeozoic‐Mesozoic relationships from a broad area of central East Timor, including:

1. (a) ‘allochthonous’ Permian rocks unconformable on metamorphic rocks

2. (b) ‘allochthonous’ Permian units interbedded with ‘autochthonous’ Permian units, and

3. (c) ‘autochthonous’ Triassic sediments stratigraphically overlying ‘allochthonous’ Permian rocks.

The model is supported by recent modifications in palaeogeographic interpretations for the Permian of north Australia (e.g. Powell, 1976; Thomas, 1976). Our observations support and extend the earlier suggestions of Grady (1975), and the resulting model is in contrast with some of the hypotheses of Audley‐Charles and his associates (as, most recently, Barber et al., 1977), Fitch & Hamilton (1974), Hamilton (1973, 1976), and Crostella (1976).

Our model involves no essentially allochthonous pre‐Cainozoic material in Timor. The Permian to Cretaceous units are envisaged as developing on the continental margin which was dominantly inactive, but affected to some extent by Late Jurassic rifting activity. Following the Pliocene collision with the Inner Banda Arc, uplift along the collision zone would have caused gravity gliding towards the south. Thus, surficial olistostrome deposits, originally from the island arc, could have eventually moved to the northern slopes of the Timor Trough, while at depth, reverse faulting could have developed as a result of gravity gliding.

We maintain that previous postulates of a pervasive, strongly imbricate structure for Timor, lack adequate substantiation in the literature. Furthermore, accounts of the tectonic development of Timor, involving large scale translation on low angle faults, are even less well substantiated.     

Insights about the structure and development of Zhongsha Bank in the South China Sea from integrated geophysical modelling

ABSTRACT

We construct a complete density transection based on the velocity structures across the Zhongsha Bank in the South China Sea. Gravity modelling of the lateral density contrasts between tectonic units helps us to determine the structural attributes and boundaries between continental blocks and deep basins. The configuration of the continent–ocean boundary (COB) around the Zhongsha Bank is mapped based on the gravity/magnetic anomaly and crustal structures. A low-density mantle is found beneath the Zhongsha Bank and the oceanic basins, and this mantle is associated with the high heat-flow background. The COB orientation is northeast-east in the north of the bank, with faulted linear structures. In further southeast, where there is a more intact crust, the COB orientation changed to north-northeast. The reconstructed density model and gravity/magnetic map indicate that the Zhongsha Bank is conjugated with the Liyue Bank by a rifted basin, where the crust had experienced localized deformation before the seafloor spreading. Because of the insufficient magmatism in the oceanic basin, the spreading ridge propagates into the weakened continental lithosphere between the two continental blocks, thus completely separating the Zhongsha Bank from the Liyue Bank. Seafloor spreading ridge jumps within the South China Sea may also be affected by the heterogeneous lithosphere beneath the continental blocks and oceanic basins.     

The Sumba enigma: Is Sumba a diapiric fore-arc nappe in process of formation?

The anomalous updomed morphological expression of Sumba island, its enigmatic lack of strong Neogene deformation and the northward morphological indentation of southern Sumbawa and Flores require explanation.The stratigraphy of Sumba may be correlated with the Cretaceous to Miocene part of the Timor allochthon. The sedimentary and eruptive rock succession in Sumba shows remarkable similarities with the allochthonous Palelo, Wiluba and Cablac deposits of Timor. In both islands the Cretaceous parts of these sequences are regarded as characteristic of fore-arc deposits built on thin continental crust.The Timor nappe is interpreted as a 5 km thick tectonic flake of the Banda fore-arc thrust onto the Australian continental margin in the mid-Pliocene collision. The postulated Sumba nappe has not yet been thrust onto the Australian margin which, in the Sumba region, has not yet converged as close to the arc as in the Timor area. The postulated Sumba nappe is interpreted as a diapiric elongated dome of the Sunda fore-arc that is being squeezed by the converging margin of Australia against the volcanic islands of Sumbawa and Flores.The absence of indications on the seismic reflection profiles for the presence of the thrust fault of the Sumba nappe may perhaps be explained by the thrusts being nearly horizontal within flat-lying strata.The Savu thrust is correlated with the probably older (pre-Late Pliocene) Wetar Suture as a major southward dipping lithospheric rupture. East of 124°E, this suture does not seem to have moved much since the mid-Pliocene collision that emplaced the nappes on Timor. However, microearthquake data suggest some activity is continuing.     

中沙地块南部断裂发育特征及其成因机制

利用最新多道地震剖面资料,结合重力、磁力、地形等地球物理资料,揭示了中沙地块南部断裂空间展布特征、断裂发育时期、断裂内部构造形变特征及深部地壳结构,并基于认识探讨了断裂的发育机制。研究结果认为,中沙地块南部陆缘构造属性为非火山型被动大陆边缘：地壳性质从西北向东南由减薄陆壳向洋陆过渡壳再向正常洋壳发育变化;Moho面埋深从中沙地块下方的26 km快速抬升到海盆的10~12 km;从中沙地块陡坡至其前缘海域的重力异常明显负异常区为洋陆过渡带,在重力由高值负异常上升到海盆的低值正、负异常的边界为洋陆边界。中沙地块南部发育有4组阶梯状向海倾的深大正断裂,主要发育时期为晚渐新世到中中新世。断裂早期发育与南海东部次海盆近NS向扩张有关,后期遭受挤压变形、与菲律宾海板块向南海的NWW向仰冲有关。该研究有助于更好认识南海海盆的扩张历史和南海被动大陆边缘的类型。     

Geophysical transects of the Labrador Sea: Labrador to southwest Greenland

In 1977 the Federal Institute for Geosciences and Natural Resources, Hannover, carried out a large scale multichannel reflection seismic survey in the Labrador Sea. This survey provided an opportunity for the direct comparison of the geologic structure of the Labrador and Greenland margins. The seismic records across the Labrador Shelf show a thick, prograding sedimentary wedge consisting of several seismic sequences onlapping an acoustic basement that dips steeply seaward. The surface of the acoustic basement is irregular below the continental slope, indicating Late Cretaceous—Early Tertiary faulting. The thick sedimentary section below the slope is divided by an unconformity, tentatively identified as Late Tertiary in age, into two seismic megasequencies which can be subdivided. The acoustic basement on the Greenland side is also strongly faulted but is overlain, in the south, by a thin sedimentary section. The sediment cover thickens on the Greenland Shelf to the north as the shelf becomes wider.As with more southerly parts of the western Atlantic margin, a positive free-air anomaly (30–50 mgal) lies landward of the shelf break off Labrador and a smaller negative anomaly follows the base of the slope. Similar, but generally narrower features are observed along the Greenland margin. West of the negative anomaly off the Greenland slope a narrow band of lower amplitude positive anomalies tends to be associated with an acoustic basement high observed in the reflection profiles. A landward negative gradient in the simple Airy isostatic anomaly across this margin suggests that the ocean—continent boundary is related to this high.Detailed magnetic measurements across the northern Labrador margin show that well-developed oceanic anomalies trending north-northwest lie east of the large Labrador Shelf gravity high, beyond the 2000 m isobath. Landward of these magnetic anomalies is a quiet magnetic zone within which the linear gravity high is parallel to the shelf break and correlates with a deep, sediment-filled basin. It is inferred that oceanic-type crust or greatly-attenuated continental crust underlies this basin and that continental crust thickens markedly westward of the gravity high over a distance of about 50 km.